This invention relates to polishing compositions for polishing, or grinding, the surface of a solid material such as wood, metal, glass and plastic materials. More particularly, this invention relates to such polishing compositions for high-precision polishing such as the polishing of end surfaces of optical fibers for telecommunication, the polishing of main surfaces of optical lenses, the texturing and polishing of magnetic hard disk substrates and the surface processing of silicon wafers.
Texturing processes for forming uniformly textured microscopic lines without scratches on the surface of a magnetic hard disk substrate used as a magnetic memory medium and polishing processes on end surfaces of optical fibers for connecting two of them together or optical fibers with an optical device, as the distance of transmission by optical fibers for telecommunication increases, are both being commonly practiced. For such high-precision polishing processes, use may be made of free abrasive particles, as well as a tape having abrasive particles attached to one of its surfaces to be pressed against the target surface, thereby causing the tape to run against the target surface. When the polishing is done by means of such free particles, the process is usually carried out by supplying a polishing composition containing abrasive particles to the target surface, and applying and moving a woven or non-woven polishing cloth or a polishing tape.
Conventionally used examples of such a polishing composition include suspensions having about 0.01-5 weight % of abrasive particles dispersed in an aqueous solution containing a surfactant at 1-20 weight %. The surfactant is used for preventing precipitation of the abrasive particles and dispersing them uniformly throughout the liquid. Polyalkylene glycol, sodium alkyl ether sulfonate, etc. are used for this purpose.
When a polishing process is carried out with free polishing particles, it is a common practice to retrieve a suitable amount of a polishing composition from a storage container. In other words, a polishing composition is usually used after it has been stored for a long period of time without even being touched. If a polishing composition is left for a long period of time without being touched in the meantime, the abrasive particles therein usually precipitate, condense and tend to form blocks having large diameters. If a polishing composition containing such condensed particles is used for a high-precision polishing process, unwanted scratches are likely to be produced on the target surface which is polished. In order to prevent such occurrences, it is a common procedure to subject the polishing composition to an ultrasonic disperser to break up such condensed blocks and to re-disperse the abrasive particles.
In other words, prior art polishing compositions were inconvenient to use because they required an extra equipment such as an ultrasonic disperser for re-dispersing condensed particles. Moreover, this requires extra manpower, involving extra work and time for the polishing and increasing the cost of the operation.
Even if an ultrasonic disperser may be used on a polishing composition which has been stored over a long period of time, condensed particles are still often observed within the polishing composition. If a polishing composition containing abrasive particles of diameters 0.3 .mu.m is left for one week and an ultrasonic disperser is used thereon, many condensed particles with diameters greater than 0.3 .mu.m are found inside the liquid, as indicated by Curve B' in FIG. 1. Since such condensed particles cause unwanted scratches on the target surface, prior art polishing compositions cannot be used reliably for a high-precision polishing process.
With the recent increase in the demand for high-capacity memory devices, the surface of a magnetic hard disk substrate must be given a uniform texturing of lines without scratches and finer than the surface roughness of the substrate after mirror polishing. There is a strong need for polishing compositions for a high-speed high-precision texturing of a substrate surface.